She was detailed in RTD's book The Writer's Tale as a 30-something northerner journalist whom the Doctor was supposed to have a romantic relationship with. Ultimately though, the character was scrapped, mostly due to Catherine Tate expressing a desire to return as Donna, though I'd wager that RTD also knew what a fucking horrible idea this whole fiasco would've been after the excruciating romance elements of the previous two series and decided to show mercy on us.
If Penny had made it to production, she would undoubtedly have become the most hated companion ever because of Tumblr fans screaming "BUT HE LOVES MUH ROSE REEEEEEEEE" and NuWho haters sneering at how the Doctor is in love with someone... again...
- She was also supposed to have a "funny mum".
- The casting director briefly considered Sheridan Smith for the role. Yup, Lucie Bleedin' Miller herself. Still, it means she'd have made it to the recasting page.
- While she was ultimately banished to the realm of the anti-canon purgatory, the bare bones of the character (i.e. her name and profession) made it into the show anyway as that black lady who Miss Foster ties to a chair and pisses on.